Pressure controllers can be electronic or pneumatic. Electronic controllers require AC or DC power and analog or digital electronic inputs. They compare the measured pressure input to a set point and provide an electronic output to a control valve or other device to bring to measured pressure to the set point.
Pneumatic controllers usually directly measure the pressure in a popeline or other vessel, compare it to the pressure setpoint, and provide a pneumatic output to a control valve or other device to bring the measured pressure to the setpoint. The pneumatic controller is often mounted on the control valve.